Traditionally, relief image plates such as those for printing presses and the like, were produced by hand and were set with movable type. This method later gave way to automatic machines, such as linotypes. In recent years, printing systems have been introduced which produce relief image plates directly from photohardenable material. By exposing such a material to actinic light through a negative image, a selective hardening is obtained which can be used as a printing surface.
Most commonly, the material used for such plates is a photopolymerizable resin. This resin must be spread in an even and uniform layer on a surface on which a negative image has been positioned. Since these resins typically have a high viscosity, it is difficult to provide an even and uniform layer in a short period of time. The longer the preparation requires, the greater the cost of the finished plate to the customer.
Most commonly, a layer of photopolymerizable material is applied by depositing a quantity of material on the platemaking surface and then spreading the material by drawing a doctor blade or nip roller along the surface.
Various mechanisms have been employed in order to assist in providing a more uniform layer of the viscous liquid photopolymer. One method which has served to assist in assuring uniform thickness in the photopolymer layer, as well as controlling the overall thickness of the layer, has been a weighted top cover. This cover is brought into position over the resin layer, released and allowed to settle on the resin layer until it comes to rest on spacing members arrayed around the platemaking surface. These spacing members hold the cover above the surface at a distance which will allow for the desired thickness of the relief image plate. This operation, which equalizes any remaining thickness irregularities in the resin layer as it assures desired thickness, is referred to as docking, since the cover actually floats on the resin layer until it comes to rest on the spacing members. It should be apparent that such a top cover could alternatively be under the influence of any uniform downward force other than gravity, or in addition to gravity, although the force of gravity is normally the preferred method and the top cover is normally weighted for that reason. Likewise, all of the directions as used herein are chosen so as to take advantage of the force of gravity on the liquid polymer system. Alternatively, the platemaking system could be oriented in a vertical plane if some means were introduced to compensate for the effects of gravity.
The operation of a weighted top cover has proven very useful in assisting to efficiently provide useful relief image plates. However, the preparation of plates by this method has taken more time than necessary because, until the present invention, a process operator would not expose the photopolymer to actinic radiation until he believed the cover had docked. The operator has to estimate the time required for docking on the basis of any number of variables, including his experience, the viscosity of the photopolymer and the thickness of the resin. If the cover had not docked in the time estimated by the operator, an unusable plate would result. On the other hand, if the operator overestimated the time required, it would result in a usable plate at the expense of a slight delay. Thus, the operator was inclined to overestimate the time required. In terms of the number of plates prepared in this manner, consistent overestimation of the necessary docking time has resulted in increased cost of operation.
Another occasional problem occurs when too much or too little photopolymerizable resin is deposited on the platemaking surface. When too little resin is deposited, there is at present a needless delay prior to exposure while the operator waits for the cover to dock. But this delay represents only a minor cost factor in this circumstance. In such a circumstance, a plate made by exposure of a layer with too little photopolymer is normally unusable. Additional costs would be incurred for the additional process time required for exposure of the photopolymer, washing out the unexposed material and examination of the plate, as well as the cost of the polymer itself. Often this problem would not be discovered until the plate was examined after washing, and several other unusable plates might have been exposed before the necessary machine adjustments could be made.
When too much photopolymer has been deposited, a result of a similar nature obtains. When the cover has not docked by the time the operator exposes the photopolymer, it is likely that the resulting plate will vary in thickness and be unusable. This will result in costs and delays similar to those set out above.